Increase vmc performance

[francesco086]

We should go for 1. and 2., but in a nicer way:

• we keep the d1, d2, and vd1 methods with the indexes, but we rename them as getD1, getD2, getVD1
• we introduce a new method called computeAllValues(double * x), which will compute and store internally in the class all the derivatives (and the WF value itself). All these values will then be accessible with the getters.

AC:

• change the code according to the new criteria
• all the unittests and example run
• update the documentation